Thiol derivatization for LC-MS analysis of microcystins in Microcystis and Planktothrix blooms from Africa and Norway

Summary

Mercaptoethanol and MEMHEG are thiols that react rapidly and efficiently with [Mdha7]- and [Dha7]-containing microcystins, adding 78 and 356 Da, respectively, to their molecular masses. LC-MS analysis of samples before and after treatment reveals characteristic mass changes for these types of microcystins, providing added confidence to the identity of putative microcystin peaks. Comparison of fragmentation spectra of microcystin analogues before and after derivatization can also facilitate identification of putative structures via the characteristic change in the masses of [Mdha]-containing fragments. [Dhb7]-containing microcystins with mercaptoethanol react more than two orders of magnitude more slowly than their isobaric [Mdha7]-containing congeners. It is therefore possible to rapidly and conveniently differentiate [Mdha7]- and [Dhb7]-microcystins in the same sample using mercaptoethanol derivatization followed by LC-MS analysis. The thiol-derivatization method provides a powerful and versatile strategy for analysis of microcystins by LC-MS, even in complex matrices and without access to MS/MS-capability. We demonstrated the potential of this strategy by analyzing [Mdha7]-, [Dha7]-, and [Dhb7]-microcystins in bloom and culture samples from Tanzania (Lake Victoria), South Africa (Hartbeespoort Dam) (both Microcystis spp.), and Norway (Lake Steinsfjorden) (Planktothrix spp.).